The present application relates to a proximity electrical switching device operated by the reciprocating output member, or piston push-rod assembly, of a hydraulic slave cylinder, such as the slave cylinder of a motor vehicle clutch hydraulic control system.
Hydraulic control apparatus for motor vehicle mechanical clutches are in wide use at the present for operating the clutch release mechanism of a motor vehicle provided with a mechanical clutch and a conventional manually shiftable transmission or gearbox. Examples of such hydraulic control systems for mechanical clutches are disclosed in U.S. Pat. Nos. 4,407,125 and 4,454,632, and in copending applications Ser. Nos. 371,958, 376,248, 477,161, 477,162, 477,159, 477,160, 537,869, 555,667, 555,666, 555,668, all assigned to the same assignee as the present application.
It has become general practice in the automobile industry to interlock the operation of a motor vehicle engine starter motor with other controls such that the starter motor is rendered inoperative unless the transmission is in neutral or park, in motor vehicles provided with an automatic transmission or, in motor vehicles provided with a foot operated clutch and a manually operated gear shift transmission, unless the transmission is in neutral and/or the clutch pedal is fully depressed.
In application Ser. No. 590,168, filed Mar. 16, 1984 for Hydraulic Master Cylinder Switch and assigned to the same assignee as the present application, master cylinder switches are disclosed that close an electrical circuit when the clutch pedal is fully depressed thus resulting in the master cylinder input member being fully retracted. Under normal operative conditions, fully depressing the clutch pedal results in transferring hydraulic fluid from the master cylinder to the slave cylinder operating the clutch release mechanism. Therefore, under normal operative conditions, full depression of the clutch pedal results in full release of the mechanical clutch, and such an arrangement, consisting in having the starter motor relay interlock circuit enabled as a result of the clutch pedal being fully depressed, has been found to be fully satisfactory unless the conduit interconnecting the master cylinder and the slave cylinder has been accidentally ruptured, or loss of hydraulic fluid has otherwise occurred, or the slave cylinder, for some other reasons, has become inoperative. Under such abnormal operative conditions, the clutch pedal may be fully depressed although the mechanical clutch could remain engaged.
Total loss of hydraulic fluid in a motor vehicle clutch control hydraulic apparatus may be detected by placing a level sensor in the hydraulic fluid reservoir associated with the master cylinder. Total or substantial loss of hydraulic fluid results in the level sensor providing a visual and/or audio indication to the driver or, in the alternative, the level sensor may be part of the starting motor relay circuit, functioning as an interlock switch. However, low hydraulic fluid level in the master cylinder reservoir does not necessarily indicate that the clutch hydraulic control apparatus is non-operative. Under conditions of low hydraulic fluid level in the reservoir, sufficient hydraulic fluid may be present in the system to maintain the apparatus in an operative condition.
It is therefore desirable to provide an interlock switch whose function is controlled by the clutch release slave cylinder, rather than by the master cylinder and which, for example, is actuated either directly by the hydraulic fluid pressure in the working chamber of the slave cylinder or by the reaction load upon the slave cylinder output member on actuation of the clutch from fully engaged to fully released, as disclosed in U.S. Pat. No. 4,621,565 assigned to the same assignee as the present application. Alternatively, the interlock switch may be such as to be closed as a result of a full stroke of the slave cylinder output member required for releasing the clutch which, in turn, requires that the switch be operated at a predetermined position of the output member corresponding to such full release of the mechanical clutch.
One advantage of hydraulic control apparatus for motor vehicle mechanical clutches is that the hydraulic control apparatus automatically compensate for progressive wear of the clutch friction linings. In a diaphragm spring disk clutch, for example, as the disk friction lining and the pressure rings wear the ends of the diaphragm fingers move outwardly, therefore pushing back the throw-out bearing. The wear is automatically compensated for as a result of the slave cylinder pushrod being progressively caused to retract further within the cylinder upon full engagement of the clutch under the action of the diaphragm spring force, thus causing in turn a progressive decrease of the volume of the working chamber of the slave cylinder, with the excess hydraulic fluid being progressively returned to the master cylinder and from the master cylinder to the hydraulic fluid reservoir. Therefore, a fixed position limit switch associated with the slave cylinder requires constant adjustment of the member causing closure of the switch upon full release of the clutch, in order to provide a foolproof arrangement.
Such switches forming part of the hydraulic slave cylinder actuatable by hydraulic fluid displacing an output member to a position fully releasing the clutch, and capable of automatically compensating for progressive change of such position in one direction as the clutch progressively wears, are disclosed in U.S. Pat. No. 4,617,855 assigned to the same assignee as the present application. Mechanical switches, however, rely for proper functioning on the engagement of two metallic contacts, one displaceable relative to the other. The contact surfaces may become oxidized or corroded, or they may become covered with dirt, or with a thin film of oil, thus preventing the establishment of a positive path for electricity from one contact to the other.